The prior art LTE-Advanced system supports carrier aggregation, where the communication between a radio network node/base station/eNodeB, and the User Equipment (UE) is facilitated by means of concurrent usage of multiple component carriers (or serving cells) in the downlink (DL) and/or uplink (UL). In the present context, the expressions downlink (DL), downstream link or forward link may be used for the transmission path from the radio network node to the UE. The expression uplink (UL), upstream link or reverse link may be used for the transmission path in the opposite direction, i.e., from the UE to the radio network node.
Furthermore, in order to divide forward and reverse communication channels on the same physical communications medium, when communicating in a wireless communication system, a duplexing method may be applied such as, e.g., Frequency-Division Duplexing (FDD) and/or Time-Division Duplexing (TDD). The FDD approach is used over well separated frequency bands in order to avoid interference between uplink and downlink transmissions. In TDD, uplink and downlink traffic are transmitted in the same frequency band, but in different time intervals. The uplink and downlink traffic is thus transmitted separated from each other, in the time dimension in a TDD transmission, possibly with a Guard Period (GP) in between uplink and downlink transmissions. In order to avoid interference between uplink and downlink, for radio network nodes and/or UEs in the same area, uplink and downlink transmissions between radio network nodes and UEs in different cells may be aligned by means of synchronisation to a common time reference and use of the same allocation of resources to uplink and downlink.
Component carriers may be located contiguously or discontiguously within a frequency band or could even be located in different frequency bands. Hence, carrier aggregation improves the spectrum utilisation for the network operator and allows higher data rates to be provided. Although carrier aggregation is defined both for FDD and TDD, UEs in the prior art system do not operate on FDD and TDD carriers simultaneously, hence there is no carrier aggregation utilising carriers with different duplexing methods. Since network operators may be in possession of both FDD and TDD carriers, it is however desirable to extend the principle to carrier aggregation of FDD and TDD carriers.
One major issue for carrier aggregation concerns the UL feedback. For DL carrier aggregation, the UE will transmit HARQ feedback, including ACK and NACK messages corresponding to the received transport blocks, which are transmitted in the Physical Downlink Shared Channel (PDSCH). In the prior art LTE-Advanced system, the HARQ feedback is transmitted either in the Physical UL Control Channel (PUCCH) on the primary cell (PCell) or in a Physical UL Shared Channel (PUSCH), which may be scheduled on any serving cell. The PUSCH may be scheduled by means of UL grants transmitted in a downlink (DL) control channel, e.g., the Physical Downlink Control Channel (PDCCH) or an Enhanced PDCCH (EPDCCH). If a PUSCH transmission has been scheduled but the UE is not capable of simultaneously transmitting the PUCCH and the PUSCH, the PUCCH will not be transmitted and the HARQ feedback will be multiplexed into the PUSCH, possibly with user data.
Data transmissions may be arranged in subframes (e.g., of 1 ms length) and a set of subframes may constitute a radio frame (e.g., of 10 ms length). For a TDD radio frame, the number of DL subframes may be larger than the number of UL subframes. Hence, an UL subframe may be used for transmitting HARQ information corresponding to multiple DL subframes. Therefore, with FDD and TDD carrier aggregation, if a TDD carrier is configured as the PCell, multiple DL subframes in the FDD carrier may be associated with one UL subframe in the TDD carrier, designated to carry the HARQ feedback for the FDD carrier and the TDD carrier.
FIG. 1 shows one example where a TDD component carrier is configured as a PCell and where a secondary cell (SCell) comprises one DL FDD component carrier and one UL FDD component carrier. Thus FIG. 1 illustrates the SCell DL HARQ timing, i.e., the timing relation between DL subframes of the SCell to an UL subframe in the PCell. For example, subframe 2 in the TDD carrier may be used to transmit HARQ feedback for subframe 1, 2, 5 and 6 of the FDD carrier. Additionally, subframe 2 in the TDD carrier may be used to transmit HARQ feedback for some subframes of the TDD carrier. Furthermore, the SCell UL scheduling timing is also shown for some of the UL subframes in the SCell, i.e., the timing relation between an UL grant transmitted in a DL control channel on the SCell and the scheduled PUSCH on the SCell.
If a PUSCH is scheduled on the PCell, HARQ feedback from the PCell and/or the SCell can be transmitted in this PUSCH. On the other hand, if a PUSCH for HARQ feedback is only scheduled on the SCell (i.e., an FDD carrier), HARQ feedback from the PCell and/or the SCell can be multiplexed into the PUSCH of the FDD carrier. However, for the FDD UL carrier, there exist more UL subframes compared to a TDD carrier and thus more opportunities for scheduling a PUSCH. At the same time, it is crucial that both the UE and the eNodeB unambiguously know how and when the HARQ feedback is carried in a PUSCH. Otherwise, the eNodeB may lose HARQ information, which will decrease the spectral efficiency of the system due to causing more retransmissions as well as introducing more signalling from the UE.
Moreover, the amount of HARQ feedback depends on how many DL subframes that contained actual transmissions. In order to determine suitable number of time-frequency resources to be used for the HARQ feedback in the PUSCH, a DL Assignment Index (DAI) can be signalled in the UL grant. The DAI may represent the total number of subframes that contained DL transmissions in the associated set of DL subframes. Also here it is crucial that both the UE and the eNodeB unambiguously know how to utilise the DAI values in order to use as few time-frequency resources as possible for the HARQ feedback, i.e., to maximize the spectral efficiency of the system.
In LTE-Advanced, carrier aggregation is performed by receiving/transmitting on a set of serving cells, wherein a serving cell comprises at least a DL component carrier and possibly an UL component carrier. A UE is always configured with a primary serving cell (PCell) and additionally also with secondary serving cells (SCells). Here, the notion of cell may not refer to a geometrical area, rather it may be regarded as logical concept. A UE is always configured with a primary serving cell (PCell) and additionally also with secondary serving cells (SCells). The PUCCH is always transmitted on the PCell.
HARQ feedback is sent in the UL (in the PUCCH or the PUSCH) in response to a PDSCH scheduled by PDCCH/EPDCCH, a Semi-Persistently Scheduled (SPS) PDSCH or a PDCCH/EPDCCH indicating SPS release. Three HARQ feedback states are used; ACK, NACK and DTX. A successful decoding attempt results in an ACK while a NACK is sent if the decoding attempt was non-successful. DTX refers to discontinuous transmission, which occurs if the UE did not receive any PDSCH, e.g., if it missed receiving a transmitted PDCCH/EPDCCH, or if there was no transmitted PDCCH/EPDCCH or PDSCH. Sometimes NACK is merged with DTX to a joint state NACK/DTX. In case of a joint NACK/DTX state, the eNodeB cannot discriminate between the NACK and DTX and would, if there was a scheduled PDSCH, need to perform a complete retransmission. This precludes using incremental redundancy for the retransmission since the eNodeB does not know whether the UE made a non-successful decoding attempt or not.
For TDD, a component carrier is configured with 1 out of 7 UL-DL configurations, defining the transmission direction of the subframes in the radio frame. A radio frame comprises DL subframes, UL subframes and special (S) subframes. The special subframes contain one part for DL transmission, a guard period and one part for UL transmission. The number of DL subframes, M, (sometimes also referred to as a bundling window) which are associated with an UL subframe for transmitting HARQ feedback is dependent on the TDD UL-DL configuration as well as the index of the specific UL subframe. In practice, the same UL-DL configuration has to be used in neighbouring cells in order to avoid UE-to-UE and eNodeB-to-eNodeB interference. However, LTE-Advanced also allows the possibility to dynamically change the UL/DL configuration. Such UEs may follow a different HARQ timing (e.g., that of another reference TDD UL-DL configuration) than that of the actually used UL-DL configuration for the transmissions.
The PDCCH/EPDCCH comprise the DL Control Information (DCI) related to the PDSCH transmission (i.e., a DL assignment) or the PUSCH transmission (i.e., an UL grant). For TDD, the DCI comprises DAI of 2 bits. When the DCI comprises a DL assignment, the DAI works as an incremental counter on a subframe basis for the number of PDCCHs/EPDCCHs/PDSCHs that were transmitted during the set of M DL subframes. With the DAI information, the UE may be able to detect whether it has missed receiving any PDCCH/EPDCCH, except for the last subframe of the set of M DL subframes. When the DCI comprises an UL grant, the DAI works as an indication of the total number of PDCCHs/EPDCCHs/PDSCHs that were transmitted during the associated set of M DL subframes and this information is utilised in order to detect whether the UE missed any transmissions and to determine the number of time-frequency resources to be used for HARQ feedback in the PUSCH. In the case of carrier aggregation, the UL DAI may represent the maximum number of subframes that were transmitted during the set of M DL subframes for all component carriers.
For FDD, the UL scheduling timing is such that an UL grant transmitted in subframe n schedules the PUSCH in subframe n+4. Moreover, a PDSCH scheduled by PDCCH/EPDCCH, a Semi-Persistently Scheduled (SPS) PDSCH or a PDCCH/EPDCCH indicating SPS release transmitted in subframe n would imply that its associated HARQ feedback is transmitted in the UL in subframe n+4. Due to the processing time in the eNodeB, a PDSCH from the same HARQ process could be retransmitted earliest in subframe n+8. The round trip time delay is thus 8 subframes, which implies that 8 HARQ processes can be used. The DL HARQ protocol is asynchronous in the DL and the HARQ process number is explicitly signalled in the DCI.
Furthermore for FDD, HARQ feedback on the PUSCH can be transmitted in any UL subframe subject to that a DL transmission in subframe n would imply that it's associated HARQ feedback is transmitted on the PUSCH in subframe n+4. The UL grant would therefore have to be transmitted in subframe n. In the prior art FDD carrier aggregation, DL subframes are associated in a one-to-one fashion to UL subframes for HARQ feedback on PUSCH and there is no bundling window (i.e., no many-to-one subframe association).
For the case where the PCell is TDD and there is at least one SCell which is FDD, it may be possible to transmit the HARQ information on PUSCH on an FDD SCell. However, it is a problem to determine which UL subframes that should contain HARQ feedback on PUSCH. It is a further problem how to arrange the DAI in the UL grants for the PUSCH.